I'm dealing with some similar issues and am on a mission to figure this out. The hops in my IPA's just aren't as pronounced as they should be. The malt is bland as well and it seems to get worse within a week. The flat sample going into the keg tastes great, but then it all goes downhill. This isn't very obvious in my stouts and other beers...but I think they too get slightly "watered down" with time. I too have tasted the other things mentioned in this thread...slightly sour / green apple like, maybe a bit medicinal, etc. But it is super obvious in my Pale Ales and IPA's.
Like all of you, I'm getting really frustrated. IPA's are my favorite beer style. I'm extremely meticulous with my cleaning and sanitation, I've listened to all of the podcasts, searched the internet for hours and hours, etc, etc. Can't believe I finally stumbled onto this thread. It is exactly what I'm experiencing.
Here is what I've tried and ruled out...
1. Oxidized Hops - I recently made a Pale Ale with brand new vacuumed sealed hops from Hop Union, ordered from Northern Brewer. Did not make a difference.
2. Oxidation in general - I don't do a secondary and go straight from the fermentor to the keg. I started purging my kegs with CO2 and am VERY careful about not splashing the beer while racking to the keg. Didn't make a difference.
3. Water & Mash pH - I've been messing with my water constantly since I started brewing. I have an RO system in my kitchen so that is what I used. Then I learned that my city treats our water with Chloromines and that RO systems can't get rid of it, so I started using Campden tablets. I started using Gypsum and Calcium Chloride to create water profiles recommended by Tasty, Martin, AJ, etc, etc. I've tried adding the salts straight to the grain, then to all of the water before brewing, to the boil, etc, etc. I make sure my mash pH is between 5.2 and 5.4. I've treated the mash with Lactic Acid and also have treated the water before mashing in. None of this has seemed to make a difference as far as this problem is concerned. In fact my last Pale Ale was right at 5.4 without any addition of Lactic Acid, so it's not the Lactic Acid either.
I recently joined our local home brew club and was really surprised that almost NOBODY treats their water with Campden Tablets OR uses RO water. Most just run their tap water through a carbon filter and don't mess with it at all...and I've tasted some really nice beers of theirs, including IPA's. So I'm going to try this next.
4. Slight Infection - Here is where things get interesting. I've become overly crazy about sanitation thanks to this problem. But having a REALLY SLIGHT / MILD infection is one thing that's been in the back of my mind. I think it might be the ONLY thing that can zap the flavor of the amount of hops that I'm using in my IPA's. I also think if my stouts had this really slight infection that it would be hard to detect, but still there...which I think I've tasted. I think this might be why they are nice and rich and malty in the beginning, but lose that complexity over time.
I made a fresh batch of Star San a few weeks ago when I brewed Rye IPA. A few days ago I kegged a batch of Belgian Wit and just for fun I tested it with my pH meter and it was 4.4. Star San is supposed to only be effective if it is lower than 3.5. So I made a fresh batch of Star San and it was 4.2!! I talked to someone at Five Star and they said that Star San can lose it's effectiveness after 1 year! They said that I could try adding more Star San to compensate, but that I can't exceed 300 ppm or it will require a "rinse" before using. The limit is 1.5 oz for 5 gallons. There is no "expiration date" on the bottle, and I have never heard or read of anyone mentioning this. Not online or anywhere else. There are several posts about how long a "Mixed" batch of Star San will keep, but not the bottle of Star San itself. How many local home brew shops have bottles of Star San sitting on the shelves for over a year before they are purchased? So this really baffled me. I have never heard anything about it. Surely if this were happening to lots of people we would have heard about it? Not sure.
So that is where I'm at with this currently. I have also never ran a brush through the serving tube of my kegs. I just soak them in PBW for 20 minutes and run it through the lines, rinse with water, and run Star San through the lines before serving.
If I ever figure this out I will definitely report back.